The present invention relates to a fine ferrite-based steel and a production method thereof. More specifically, the present invention relates to a fine ferrite-based steel which is a ferrite-based steel used in various forms such as steel bar, steel section, steel sheet, and steel plate as texture steels, etc., and has a high strength and a long fatigue life, and to a production method thereof.
Hitherto, as a strengthening method of a stool material, a solid solution strengthening method, a strengthening method by a secondary phase by forming a composite with martensite, etc., a deposition strengthening method, and a strengthening method by fining the crystal grains are known. Among these methods, as a method of increasing both the strength and the toughness and improving the strengthxe2x80xa2ductility balance, the method of strengthening by fining the crystal grains is the most excellent method. Because the method does not require the addition of an expensive element such as Ni, Cr, etc., for increasing the hardenability, it is considered the production of a high-strength steel material at a low cost is possible. From the view point of fining the crystal grains, it is expected that when in a texture steel, the grain sizes of the crystals of martensite are fined to 2.5 xcexcm or smaller, the strength is suddenly increased. However, in the grain sizes of about 5 xcexcm obtained by a conventional thermo-mechanical treatment technique, it is the present state that though a high strength is obtained, a large increase of the strength has not yet been obtained.
On the other hand, a controlled rollingxe2x80xa2accelerating cooling technique was an effective method for obtaining fine ferrite. That is, by controlling the accumulated deformation in the austenite unrecrystallization region and the cooling rate after that, a fine structure has been obtained. However, the limit of the ferrite grain size obtained was at most 10 xcexcm in an Sixe2x80x94Mn steel and 5 xcexcm in an Nb steel. On the other hand, as described in Japanese Patent Laid open Nos. 5-123823 and 59-205447, Japanese Patent Publication Nos. 62-39228, 62-5212, and 62-7247, it is reported that in the case of applying a reduction of at least 75% of the total area-reduction ratio in the temperature range of Ar1 to Ar3+100xc2x0 C. including a 2-phase region and thereafter cooling 20 K/s or higher, ferrite grains of from about 3 to 4 xcexcm are obtained. However, quenching of 20 K/s or higher is a means which can be realized only when the thickness of a steel sheet is thin and is only a non-practical means which cannot be widely realized as a production method of conventional welding steels. Also, about the large deformation itself, in rolling, it is generally difficult to carry out a large reduction exceeding 50% by one pass in an austenite low-temperature region against the deformation resistance and the gripping restriction of a roll. Also, for the accumulation reduction in an unrecrystallized region, 70% or higher is generally necessary and it is difficult by temperature lowering of a steel sheet.
Also, on the other hand, in xe2x80x9cTekko No Kesshoryu Chobisaika (Super Fining of Crystal Grains of Iron and Steel)xe2x80x9d, edited by The Iron and Steel Institute of Japan (1991), page 41, by changing the view point, by recrystallizing a bainite structure, a fine ferrite texture is obtained. However, even if the components optimization is achieved, the recrystallization temperature cannot be lowered and the growth of the ferrite grains is not lowered, and the ferrite grain size of less than 5 xcexcm is not obtained.
Thus, an object of the present invention is to overcome the limits of conventional techniques as described above and to provide a novel steel having a ultra-fine ferrite structure of 2.5 xcexcm or less, which has never been known, for far largely increasing the strength thereof and having excellent characteristics such as the greatly long fatigue life, etc.
It has now been found that the object described above has been achieved by the present invention as set forth hereinbelow.
That is, a first aspect of the present invention is to provide a fine ferrite-based steel comprising a ferrite-based steel obtained by work-induced recrystallizing from a martensite steel after heating to a temperature of from 500xc2x0 C. to Ac1, wherein the mean ferrite grain size is not larger than 2.5 xcexcm.
A second aspect of the present invention is to provide a fine ferrite-based steal of the first aspect wherein the martensite steel is a steel obtained by heating a steel material to a temperature range of from Ac3 to 1,350xc2x0 C. and quenching from an austenite region after working or without working.
A third aspect of the present invention is to provide a fine ferrite-based steel of the first or second aspect wherein the work-induced recrystallization is carried out by working of a reduction ratio of at least 50%.
A fourth aspect of the present invention is to provide a fine ferrite-based steel of first to third aspects wherein the martensite steel is obtained from a steel material containing, as the chemical composition:
C. 0.001 to 0.80 mass %,
Si: not more than 0.80 mass %,
Mn: 0.8 to 3.0 mass %, and
Al: not more than 0.10 mass %,
with the rest being Fe and unavoidable impurities.
A fifth aspect of the present invention is to provide a fine ferrite-based steel of the fourth aspect wherein the martensite steel is obtained from the steel material further containing at least one kind of:
Cu: 0.05 to 2.5 mass %,
Ni: 0.05 to 3 mass %,
Ti: 0.005 to 0.1 mass %,
Nb: 0.005 to 0.1 mass %,
V: 0.005 to 0.1 mass %,
Cr: 0.01 to 3 mass %,
Mo: 0.01 to 1 mass %,
W: 0.01 to 0.5 mass %,
Ca: 0.001 to 0.01 mass %,
REM: 0.001 to 0.02 mass %, and
B: 0.0001 to 0.006 mass %,
in addition to the compositions described in the fourth aspect.
A sixth aspect of the present invention is to provide a fine ferrite-based steel, characterized in that the steel has a fine ferrite structure wherein at least 60% of the ferrite grain boundary is a large angle grain boundary of at least 15xc2x0, and the mean grain size is not larger than 5 xcexcm.
A seventh aspect of the present invention is to provide a production method of a fine ferrite-based steel, which comprises working a steel material capable of forming a ferrite phase by working to cause recover and recrystallization and producing a fine ferrite-based steel having a fine ferrite structure wherein at least 60% of the ferrite grain boundary is a large angle grain boundary of at least 15xc2x0 and the mean grain size is not larger than 5 xcexcm.
An eighth aspect of the present invention is to provide a production method of a fine ferrite-based steel of the seventh aspect wherein the steel material is worked at 50% or more by the total working amount.
A ninth aspect of the present invention is to provide a production method of a fine ferrite-based steel of the seventh or eighth aspect wherein working is carried out by at least two passes and in the at least optional two passes, the reducing direction or the rolling direction differs from each other.
A tenth aspect of the present invention is to provide a production method of a fine ferrite-based steel of the ninth aspect wherein in the at least optional two passes, each total reduction ratio or total rolling ratio is at least 29%.
An eleventh aspect of the present invention provides a production method of a fine ferrite-based steel of the seventh to tenth aspects wherein the structure before working is martensite or annealed martensite.